Transparent polycrystalline body with high ultraviolet transmittance

ABSTRACT

A transparent polycrystalline body having high ultraviolet transmissivity is provided. The body is preferably a sintered magnesia-alumina spinel body having a transmittance of at least 85 percent in the wavelength range from 0.3 microns to 0.8 microns and a transmittance at some wavelength in the wave length range between about 0.2 and about 0.4 microns of at least about 86 percent and a porosity of less than about 0.001 percent. The sintered body is produced by a two-step method comprising forming a closed porosity body from a spinel powder and subjecting the closed porosity body to a hot isostatic pressure treatment. The resulting body has high transmissivity of electromagnetic radiation in the range between about 0.2 microns and about 6 microns, is resistant to abrasion or erosion, does not substantially deteriorate after exposure to ultraviolet light, and has high strength.

This is a continuation of co-pending application Ser. No. 07/047,403 filed on May 6, 1987 now abandoned.

FIELD OF THE INVENTION

The present invention relates to a transparent polycrystalline body with high transmittance and, particularly, a polycrystalline body of sintered spinel with high transmittance in the ultraviolet wave lengths.

BACKGROUND INFORMATION

In a number of applications, there is need for a strong, tough, transparent window material having a high transmissivity throughout a wide range of electromagnetic radiation. Such a window material is useful, for example, for covering a port used in connection with instrumentation for detecting electro-magnetic radiation. Materials which have been used for producing a transparent body include metal fluorides, particularly magnesium fluoride (U.S. Pat. No. 3,589,880 issued June 29, 1971 to Clark; U.S. Pat. No. 3,294,878 issued Dec. 27, 1966 to Carnall, Jr., et al.; U.S. Pat. No. 3,431,326 issued Mar. 4, 1969 to Letter), aluminum oxynitride (U.S. Pat. No. 4,520,116 issued May 28, 1985 to Gentilman, et al.), aluminum niobate or tantalate (U.S. Pat. No. 4,047,960 issued Sept. 13, 1977 to Reade), and solid solutions of alumina, silica and other oxides (U.S. Pat. No. 4,009,042 issued Feb. 22, 1977 to Wittler) and alumina, with minor amounts of spinel (U.S. Pat. No. 3,026,210 issued Mar. 20, 1962 to Coble).

Previous methods of ceramic preparation which have included a hot-press or closed-porosity step followed by a hot isostatic press step have included U.S. Pat. No 4,461,750 issued July 24, 1984 to Chess, et al., U.S. Pat. No. 4,524,138 issued June 18, 1985 to Schwetz, et al., and U.S. Pat. No. 3,853,973 issued Dec. 10, 1974 to Hardtl, et al.

Methods have also been developed for production of transparent bodies substantially from a magnesiaalumina spinel. U.S. Pat. No. 3,974,249 issued Aug. 10, 1976 to Roy, et al., U.S. Pat. No. 3,768,990 issued Oct. 30, 1973 to Sellers, et al., U.S. Pat. No. 3,531,308 issued Sept. 29, 1970 to Bagley. Polycrystalline bodies of spinel are, in general, more easily formed than single-crystal or fusion-cast spinel or sapphire.

Previous materials and methods for production of a sintered transparent window have suffered from a number of difficulties. These materials have been deficient in transmittance in certain wavelength ranges, particularly ultraviolet ranges, for example, wave lengths from about 0.2 micrometers (microns) to about 0.4 microns, as well as visible and infrared wave lengths up to about 6 microns.

Previous materials were susceptible to abrasion or erosion, for example, from high velocity impaction of dust or sand particles or rain or cloud droplets.

Previous materials were often unstable under conditions of long exposure to ultraviolet light such that exposure to sunlight or to ultraviolet light with an intensity of about 700 microwatts/cm², on the order of 0.25 hours or more caused a reduction of the transmittance properties of the material.

Previous materials have been difficult to form with the desired structural strength. In some applications it is desired to produce a transparent window which can withstand mechanical stress on the order of a pressure of about 15 psi (0.1 MPa), but which will preferably rupture when subjected to a pressure of about 25 psi (0.17 MPa) or more.

Certain previous materials, e.g. MgO, are hygroscopic and become cloudy upon exposure to moisture, rendering the optical qualities of the material unacceptable.

Previous production methods have been costly to practice and have required a number of difficult steps making the windows impractical to produce in quantity.

Accordingly, a number of advantages would be realized by methods and materials for producing a transparent window having high transmissivity, particularly in the ultraviolet spectral region, as well as in the infrared region (e.g., about 3 to 5 microns), high resistance to erosion or abrasion, low susceptibility to deterioration from ultraviolet light, a desired degree of strength, low cost and ease of manufacture.

SUMMARY OF THE INVENTION

According to the present invention, a polycrystalline body is produced having a high in-line transmittance and, particularly, a transmittance greater than that depicted by line A in FIG. 1. The body preferably comprises magnesia-alumina spinel. The body preferably has, in its uncoated state, and with a thickness of about 1.88 mm, a transmittance not less than about 80 percent, preferably not less than about 85 percent, and more preferably not less than about 87 percent, in all wavelengths from 0.3 to 0.8 microns and not less than about 60 percent, preferably not less than about 72 percent, and more preferably not less than about 77 percent, at some wavelength from 0.2 to 0.3 microns. The material retains at least about 90 percent and preferably at least about 99 percent of its original transmissivity after as much as 240 hours or more of exposure to ultraviolet light of a wave length between about 0.2 and about 0.4 microns. The sintered body has a flexural strength of at least about 15,000 psi (about 100 MPa) and preferably at least about 20,000 psi (about 140 MPa), most preferably at least about 28.000 psi (about 200 MPa).

The sintered transparent body can be produced by a two-step process involving forming a closed porosity body in a first step and reducing residual porosity in a second step. The method comprises forming a powder, preferably of spinel, and preferably having less than 0.005 weight percent of certain impurities. The powder is formed into a closed porosity body. The closed porosity body can be formed by, e.g., hot pressing or pressureless sintering. The closed porosity body is then hot isostatic pressed at a temperature of at least about 1400° C. under a pressure of at least about 15,000 psi (about 100 MPa). The sintered body produced by this method has a porosity less than about 0.001 percent, and, in a thickness of about 1.88 mm, an in-line transmittance greater than or equal to that depicted by line A in FIG. 1.

A method of producing transparent spinel is disclosed in copending U.S. patent application titled "Method for Producing Transparent Polycrystalline Body With High Ultraviolet Transmittance", Ser. No. 047,428 by applicants on even date herewith. A dome and window produced from transparent spinel is disclosed in copending U.S. patent application titled "Dome And Window for Missiles And Launch Tubes With High Ultraviolet Transmittance", Ser. No. 047,422 filed by applicants on even date herewith.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a preferred and a more preferred in-line transmission of polycrystalline bodies made according to the present invention;

FIG. 2 depicts the transmission curves of FIG. 1 in greater detail;

FIG. 3 illustrates spectral transmission curves of single crystal and hot-pressed spinel bodies made by methods other than those of the present invention;

FIG. 4 illustrates spectral transmission curves of calcium aluminate glasses;

FIG. 5 illustrates spectral transmission curves of quartz crystal and silica and fused silica glass;

FIG. 6 illustrates spectral transmission curves of MgO and MgF₂ ;

FIG. 7 illustrates spectral transmission curves of zinc alumino germanate and germanate glasses;

FIG. 8 illustrates spectral transmission curves of SrF₂, BaF₂, and zinc sulfide;

FIG. 9 illustrates spectral transmission curves of sapphire, AION, and yttria;

FIG. 10 illustrates spectral transmission curves of a number of samples of material made according to the present invention;

FIG. 11 depicts a phase diagram for the MgO--Al₂ O₃ -- spinel system; and

FIG. 12 illustrates a relationship between optical transmission and porosity used to obtain porosity from measurements of optical transmission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, there is provided a sintered polycrystalline body which is highly transparent, particularly in the ultraviolet region between about 0.2 and about 0.4 microns wavelength. The sintered bodies produced according to the present invention have an in-line transmittance greater than or equal to the transmittance depicted by line A in FIGS. 1 and 2. The materials of the present invention are capable of a transmittance even greater than that depicted by line A in FIGS. 1 and 2 and, accordingly, the present invention also includes producing a sintered body with a transmittance greater than or about equal to that depicted by line B in FIGS. 1 and 2. The sintered body of the present invention also has good transmittance in the infrared wavelength region, having an in-line transmittance per 1.88 mm thickness at all wavelengths from 3 microns to 5 microns of not less than about 73 percent, preferably not less than about 76 percent, and, at all wavelengths from about 3 microns to 4 microns of not less than about 81 percent, preferably not less than about 84 percent.

The body preferably has, in its uncoated state and with a thickness of about 1.88 mm, a transmittance not less than 85 percent in all wavelengths from 0.3 to 0.8 microns and not less than about 72 percent at some wavelength from 0.2 to 0.3 microns. Further, the body preferably has a transmittance not less than about 73 percent in all wavelengths from about 3 microns to about 5 microns and has a transmittance of at least about 86 percent at some wavelength from about 0.2 to about 0.4 microns. The body preferably has, in its uncoated state and with a thickness of about 1.88 mm, a transmittance greater than about 30 percent, preferably greater than 40 percent, and most preferably greater than about 45 percent, at a wavelength of about 0.2 microns and a transmittance greater than about 84 percent and preferably greater than about 85 percent at a wavelength of about 0.4 microns.

Lines C through E of FIG. 3, provided for purposes of comparison, depict transmittance reported for spinel bodies produced by previous methods. Lines F through W of FIGS. 4-9 depict transmittances reported for sintered bodies made of non-spinel materials as described in Table 1, and are provided for comparison purposes.

                  TABLE 1     ______________________________________     Curve   FIG.     Material     Thickness (mm)     ______________________________________     C       3        single-crystal                                   1.6                      spinel     D       3        hot-pressed  --                      spinel     E       3        hot-pressed  4.9                      spinel     F       4        calcium-aluminate                                   2                      glass     G       4        calcium-aluminate                                   2                      glass     H       4        calcium-aluminate                                   2                      glass     I       5        high-silica glass                                   1     J       5        fused silica 10     K       5        quartz crystal                                   10     L       6        MgF.sub.2    2     M       6        MgF.sub.2    --     N       6        MgO          2.5     O       7        zinc alumino 1                      germanate     P       7        germanate glass                                   2     Q       7        Ge--mullite  0.76                      62% Al.sub.2 O.sub.3     R       8        SrF.sub.2    2.9     S       8        BaF.sub.2    2.9     T       8        zinc sulfide --     U       9        sapphire     1.5     V       9        AlON         1.5     W       9        yttria       1.5     ______________________________________

A sintered body produced according to the present invention comprises spinel and preferably contains more than about 99.9 weight percent spinel. Spinel is a useful material for optical applications such as instrument ports because, among other reasons, spinel is non-birefringent, has a cubic structure, and is substantially non-hygroscopic. The sintered transparent body has a porosity of less than 0.01 percent, preferably less than about 0.001 percent by volume and an average pore radius less than about 0.1 microns. Porosity in such low range is difficult to measure by ordinary displacement methods. Porosity can be measured using electron micrographs of the sintered body. Another method of determining porosity is to calculate porosity from measured light transmittance, e.g. by using the graphical relationship depicted in FIG. 12, which applies generally to material such as the material which is the subject of this invention.

The polycrystalline body of the present invention has a scatter of less than about 9×10⁻³, preferably less than about 8×10⁻³, and most preferably less than 7×10⁻³ at a wavelength of about 0.6471 microns.

The sintered body can be polished to a scratch-dig size of less than about 20--20. The scratch-dig size of 20--20 represents a maximum scratch size of 0.020 mm (0.0008 inches) in width and a maximum dig size of 0.20 mm (0.008 inches) in diameter. In this context, a scratch is a defect which is long and thin in nature and a dig is a defect which is basically round. The meaning of scratch-dig size is explained fully in Mil-0-13830, pp. 32-34, incorporated herein by reference "MIL-0-13830 states:

"Surface quality is specified by two numbers separated by a hyphen, e.g., 60-20. The first number is the maximum "scratch number" and the second number is the maximum "dig number." Scratches are classified as defects which are long and thin in nature, and digs are defects which are basically round. The smaller the number in both instances, the better the surface quality. Surface quality specifications should also integrate defects over the surface of the part. Therefore, looser tolerances should be considered for larger parts."

The sintered body has an average grain size less than about 150 microns. The sintered body has an average defect radius of less than about 0.005 inches (about 0.12 mm).

The sintered body of the present invention provides a relatively low emissivity in the infrared wavelength regions at elevated temperatures. High emissivities interfere with optics measurements and can be detrimental to optical guidance systems such as situations in which missile nose cones are aerodynamically heated during flight. Table 2 provides a comparison of emissivities of hot-pressed spinel with those of sapphire at various temperatures. Although the hot-pressed spinel represented in Table 2 is not made according to the claimed invention, since emissivities are primarily material-dependent, it is believed emissivities of spinel material made in accordance with the present invention has emissivities no greater than about those shown in Table 2 for hotpressed spinel.

                  TABLE 2     ______________________________________     Emissivity at 4.5 and 5.0 Microns     Temp.  Emissivity at 4.5 microns                                 Emissivity at 5.0 microns     (°C.)            spinel    sapphire   spinel  sapphire     ______________________________________      30    0.078     0.051      0.157   0.199     200    0.091     0.074      0.203   0.268     400    0.118     0.119      0.280   0.370     600    0.151     0.168      0.361   0.477     800    0.194     0.229      0.461   0.587     1000   0.247     0.311      0.559   0.686     1200   0.303     0.381      0.664   0.777     1400   0.381     0.468      0.788   0.848     ______________________________________

The sintered body of the present invention has a flexural strength, measured according to the ASTM F417 test, measured at 25° C., of at least about 15,000 psi (100 MPa), preferably at least about 20,000 psi (140 MPa), and most preferably at least about 23,000 psi (about 150 MPa). The sintered body has a tensile strength, measured according to the ACMA #4 test of at least about 12,000 psi (about 80 MPa). The material, however, is not so strong that it will resist bursting at a desired level. The sintered body of the present invention, when provided in a 3 inch (7.5 cm) diameter disk having a thickness of about 0.06 inches (1.5 mm), bursts when subjected to a pressure of more than about 5 psi (about 0.17 MPa).

The sintered body of the present invention is resistant to abrasion or erosion, and particularly, when subjected to dust, sand or water droplets at a velocity of about 500 meters per second, undergoes substantially no degradation of transmittance properties. The sintered body of the present invention has a Knoop hardness, measured according to the ASTM E18 test of greater than 1350 kg/mm², preferably greater than 1500, and most preferably about 1700.

The sintered body of the present invention is resistant to degradation under high temperature conditions and, in particular, has a high melting point, preferably above 2000° C.

The sintered body of the present invention is stable under exposure to ultraviolet light, and particularly, experiences a loss of transmissivity of less than about 1 percent after being subjected to sunlight or at a distance of about 6 inches (15 cm) to ultraviolet radiation of an intensity of about 700 microwatts/cm² having a wave length of about 0.254 microns for about 240 hours.

The sintered body of the present invention is chemically and thermally stable. The polycrystalline material of the present invention has a solubility of less than 10⁻³ g/100 g water at room temperature. The polycrystalline body has substantially no solubility in an acidic aqueous solution having a pH of between about 7 and about 2. There is substantially no degradation of the body upon exposure to oxidizing materials and there is substantially no degradation of the body upon exposure to temperatures of about 500° C. In this context, substantially no degradation means that the strength, hardness and optical qualities of the material are substantially preserved. In particular, upon exposure to oxidizing materials or to temperatures of about 500° C., the body possesses a knoop hardness of at least about 1038 kg/mm², a flexure strength of at least about 15,00 psi, and in-line transmittance per 1.88 mm thickness in all wavelengths in the wavelength range from about 0.2 microns to about 0.4 microns of not less than the transmittance shown by Line A in FIG. 2.

For comparison purposes, Table 3 shows a number of physical properties reported for other optical materials.

                                      TABLE 3     __________________________________________________________________________                           550-                           450° C.                                750° C.                           Resis-                                Resis-    Thermal                           tance                                tance     Conduc-                                                Melting     4-point    Knoop      to   to        tivity                                                or     Flexure    Hardness                      Fracture                           Thermal                                Thermal                                     Tensile                                          (10-4 cal./                                                Softening     Strength   200° C.                      Strength                           Stress                                Shock                                     Strength                                          cm sec                                                Temperature     (MPa)      (kg/mm2)                      (MPa)                           (K/E)                                (w/cm)                                     (MPa)                                          °C.)                                                °C.)     __________________________________________________________________________     Previous           103  1390  193  11   10   110  330   2135     hot-pressed     spinel     MgF2  100  576   151  3.2   9              1255     MgO   140  900   158  8    11        600   2800     AL2O3      220   344  21             600   2030     Y2O3  160  800   193  7     ZnS        356   103  6     Fusion-           175  1750     cast spinel     Alpha      2400     Si3N4     AlON  305  1950  206                       2140     Germinate  460     glass     __________________________________________________________________________

For purposes of comparison, absorption coefficients of other materials are given in Table 4, in which "HP" indicates hot-pressed.

                                      TABLE 4     __________________________________________________________________________     Absorption Coefficients (cm.sup.-1)             Wavelength (microns)     Material             3.0   4.0  4.5  5.0  5.5 6.0     __________________________________________________________________________     Temperature = 17-25° C.     Spinel (HP)             .0007 .0332                        .135 .465 1.16                                      3.07                             -.8  -2     Sapphire             .0013 .061 .252 .81  2.07                                      5.66     AlON               .5   1.6     Zirconia                     0.5 1.5     Yttria  .0001 .0003                        .0021                             .001 .037                                      .11                                      -.15     Temperature = 127° C.     Spinel (HP)             .0010 .046 .186 .614 1.51                                      3.38     Sapphire             .0019 .083 .333 1.6  2.63                                      6.95     Yttria  .001  .0006                        .0038                             .0017                                  .061                                      .186     Temperature = 227-250° C.     Spinel (HP)             .0015 .0646                        .243 .7-  1.95                                      4.74                             .802 -2.3     Sapphire             .0028 .113 .437 1.3- 3.3 8.48                             1.4  -3.4     AlON               0.9  2.4     Zirconia                     0.9 2.4     Temperature =  477-500° C.     Spinel (HP)             .0038 .135 .475 1.3- 3.4 7.68                             1.46 -3.7     Sapphire             .0069 .233 .826 2.1- 5.1 13.38                             2.46 -5.65     AlON    1.3   3.7     Zirconia                     2.0 4.5     Yttria  .0001 .003 .019 .072 .23 0.6     Temperature = 727° C.     Spinel (HP)             .008  .244 .809 2.35 5.3 11.19     Sapphire             .014  .417 1.38 2.93 8.6 19.15     Yttria  .00011                   .009 0.42 .15  .45 1.13     __________________________________________________________________________

The method preferred for producing the transparent sintered body will now be described. According to the method of the present invention, a magnesia-alumina spinel powder is provided. The spinel powder is of high purity, and particularly, has less than about 5 ppm by weight, preferably less than 3 ppm, of oxides of any of the following elements: Cr, Cu, Fe, Ga, Mn, Pb, Ti, V, Zn, and has less than 50 ppm, preferably less than 30 ppm of the total of all the above oxides. The powder preferably has less than 1,000 ppm, preferably less than 10 ppm of carbon, less than 10 ppm, preferably less than 5 ppm of calcium, less than 30 ppm, preferably less than 5 ppm of sodium, less than 500 ppm, preferably less than 100 ppm of sulfur, less than 1,000 ppm, preferably less than 50 ppm of silicon.

The magnesia-alumina powder is a mixture of a powder of MgO and a powder of Al₂ O₃. The mixture can be in any weight ratio of Al₂ O₃ to MgO approximately falling in the range indicated by "spinel SS" in FIG. 11. Preferably, the molar ratio of Al₂ O₃ to MgO in the powder is about 1:1 plus or minus about 5 M/0. The powder preferably has a surface area of between about 10 and about 15 m² /g. The average particle size of the powder is substantially submicron and preferably between 40 and 50 percent of the particles have a diameter less than about 1 micron.

The powder is used to form a closed porosity body. The formation of a closed porosity body can be accomplished by hot pressing or pressureless sintering. According to the hot pressing method, the powder is mixed with an amount of hot-pressing aid, e.g., between about 0.5 weight percent and about 3 weight percent, preferably about 11/2 weight percent of LiF.

The mixed powder is loaded in a hot press die. The powder compacts substantially upon pressing. The thickness desired for the hot pressed parts preferably will include an allowance for grinding. For example, a finished 0.3 inch (8 mm) thick disk may require the addition of about 0.015 inch (about 0.3 mm) for grind allowance.

The hot press die may be made of graphite, for example, in the form of a cylindrical annulus. A ctlindrical bottom die is placed in the bottom of the hot press die. All surfaces of the dies which will be exposed to the spinel are coated with a graphite foil.

According to one method of hot pressing, a single window is produced for each hot pressing operation. According to another hot pressing method, a number of windows are hot pressed simultaneously such as by vertically stacking two or more layers of spinel powder, separated by a parting medium such as graphite or boron nitride. When several windows are hot pressed simultaneously, the spinel layers are preferably separated by graphite spacers sufficiently thick to prevent fracture of the spacers. Spacers of about 1/2 inch (1.2 cm) thickness are operable, although thinner spacers may also be operable. Products can be produced in a number of shapes including disks and domes.

After the powder and any parting medium is placed into the die, a top punch is placed into the top of the die. For disks, the top punch is a cylindrically-shaped punch and for domes, the top punch is preferably a male punch. The prepared and filled die is cold pressed at about 100 psi (0.7 MPa). The cold-press pressure must be sufficient to provide for ease of handling and also assists in slightly compacting the powder prior to placing the die in the main press. The cold press pressure must not, however, be so high that closed porosity results.

The cold-pressed body can be treated to fully or partially remove impurities such as conducting in a vacuum bake at about 1100° C. to about 1300° C., preferably about 1200° C. to remove volatile impurities. Alternatively, the impurities can be removed during the step of forming closed porosity.

After cold pressing, the die is placed in a quartz cylinder, separated therefrom by an insulating material such as graphite felt. The cold pressed bodies in the die are then subjected to hot pressing. The hot press step can be achieved in an inert gas atmosphere, but is preferably conducted under a vacuum. The actual vacuum which is achieved during hot pressing will depend in part upon the rate at which impurities or other materials out-gas. Apparatus capable of producing about 60 microns of vacuum will suffice. Note that in the discussion that follows with regard to the hot press step, the temperatures are those measured at the top surface of the die by a direct optical method. These temperatures are not necessarily equal to the temperatures of the spinel. Before any pressure is applied, the die is first heated to about 1125° C. Heating is preferably achieved by an induction heating method. The spinel is held at about 1125° C. for about 30 minutes. The pressure and temperature are then both linearly increased over a period of about 1 to 5 hours. The rate of temperature rise and the rate of pressure rise are selected such that the target temperature is never reached substantially before the target pressure is reached. For disks, the target temperature is at least about 1400° C., preferably about 1410° C. The target pressure is at least about 2000 psi (about 13 MPa), preferably about 2200 psi (about 15 MPa). For domes, the target temperature is at least about 1395° C. and preferably about 1460° C., and the target pressure is at least about 2000 psi (about 13 MPa), preferably about 3200 psi (about 22 MPa). A temperature which is too high will result in a substantial amount of sublimation of the spinel. A temperature which is too low may result in decreased transparency of the product. Pressures higher than the target pressure are believed to be operable but cause excessive wear of the dies. Pressures lower than the target pressure result in compaction less than that desired and inferior optical clarity. During hot pressing, the spinel normally loses about 31/2 weight percent to sublimation.

The spinel is held at the target pressure and temperature for a period of time. The period of time for disks is about 2 to 5 hours. The period of time for domes is about 4 hours.

Following holding at the target temperature and pressure, the heating and pressure are reduced. Cooling occurs in about 12 to 14 hours. When domes are being pressed, after the die has cooled to about 1000° C., it is preferred to reduce the vacuum and to fill the die with a noble gas such as argon.

Another method for producing a closed porosity body is by pressureless sintering. Pressureless sintering, as compared to a hot press process, permits easier control of grain growth during the closed porosity formation step and is more useful in formation of parts having complex geometry. Hot-pressing is, in general, not as suitable for formation of complexgeometry parts. According to this method, the spinel powder is mixed with an amount of binder such as about 5 weight percent of carbowax, and may optionally also be mixed with materials such as surfactants, or lubricants, such as neofat. The powder mix is then compressed at substantially room temperature or is slip cast at room temperature to produce a green body. The green body is then exposed to a temperature, such as greater than about 1800° C. preferably at substantially sub-atmospheric pressure, e.g., about 100 psi (0.7 MPa). The atmosphere during said pressureless sintering can be hydrogen or vacuum.

The closed porosity body typically has some amount of residual porosity. The hot pressed body typically has a porosity of about 0.01 volume percent or more. The pressureless sintered body typically has porosity of about 1 volume percent or more. In order to produce a body with the desired characteristics, the body is subjected to a second treatment step to reduce or eliminate the residual porosity. According to the present invention, the second step is a hot isostatic pressure treatment. The closed-porosity bodies are preferably inspected for visually-apparent defects and for minimum ultraviolet transmission such as about 65-85 percent transmission of an ultraviolet source having an intensity peak at a wavelength of about 0.25 microns. The parts, preferably separated by graphite spacers, are loaded into a hot-isostatic pressing chamber. The hot isostatic treatment includes heating the body to a target temperature of about 1500° C. to 2000° C., preferably less than about 1800° C., preferably at least about 1500° C. Heating can be accomplished by a resistance heater furnace. The maximum temperature is achieved in an atmosphere which has been pressurized to at least about 15,000 psi (about 100 MPa), preferably more than 25,000 psi (about 170 MPa), and most preferably at least about 29,500 psi (about 205 MPa). The hot isostatic pressure atmosphere is preferably argon or helium. A number of rates of pressurization and heating and depressurization and cooling are operable. In one preferred embodiment, the chamber is pressurized to about 5,000 psi (about 34 MPa) without heating. The temperature and pressure are then increased linearly such that the target temperature and target pressure are arrived at essentially simultaneously. The heating is at a rate of between about 5° C. per minute and about 25° C. per minute, preferably about 15° C. per minute. The target temperature and target pressure are maintained for a period between about 1/2 hour and about 5 hours, preferably about 21/2 hours. In general, there is an inverse relationship between temperature and hold time, i.e., at higher temperatures, a shorter hold time will be operable. After the hold time or "soak" period, a controlled cool-down is used. The pressure is decreased linearly during the cool-down period at such a rate that when the temperature reaches 600° C., the pressure will be about 10,000 psi (about 70 MPa). The preferred cooling rate is about 5° C. per minute down to the temperature of 600° C. Too rapid cooling, such as more than about 15° C. per minute, may cause thermal shock. After a temperature of 600° C. has been achieved, the furnace is turned off. The pressure, during this period, is decreased linearly at a rate such that when the temperature is 300° C., the pressure is about 7,500 psi (about 50 MPa). At this point, the pressure is allowed to freely vent to the atmosphere and the parts are allowed to cool to a temperature at which they can be handled. Normally, there will be no weight loss of the spinel parts during the hot-isostatic pressing procedure.

In one embodiment of the process, the two steps of the process are accomplished without substantial cooling of the body between steps. According to this method, a formed but unfired compact is sintered to a closed porosity state in a vessel at about atmospheric pressure in an H₂ atmosphere or a sub-atmospheric pressure in a vacuum atmosphere, and at a temperature of about 1400° C. Because this process does not require a hot-press step to form the closed porosity body, it can be used in formation of bodies having complex geometry. Hot press techniques are generally not suitable for forming complex geometry bodies. After the compact is sintered to a closed porosity state, the vessel is pressurized to at least about 20,000 psi (about 140 MPa), without substantial cooling of the body, at a temperature of at least about 1400° C. to accomplish the hot isostatic pressure step of the process and reduce or eliminate residual porosity of the body.

Following the hot isostatic pressing step, the body is cooled and is subjected to further steps to modify the surface characteristics of the body. These further steps can include grinding and polishing, application of an anti-reflection coating and metallizing.

The transparent body is particularly useful as an optical element in electro-optical systems that will be exposed to extreme environments such as high-abrasion, high-heat, high-stress or high-corrosion environments. Applications include missile or guided artillery components including guidance system components such as domes, windows, lenses, and prisms, as well as armor and fuse covers.

EXAMPLES Example 1

Three sintered bodies of magnesia-alumina spinel were formed and the properties thereof tested. All three bodies were formed from a powder having less than 5 parts per million by weight of oxides of any of Cr, Cu, Fe, Ga, Mn, Pb, Ti, V, and Zn and having less than 50 ppm of the total of all the above oxides. The powder had less than 1000 ppm of carbon and less than 10 ppm of calcium, less than 30 ppm sodium, less than 500 ppm sulfur, less than 1000 ppm silicon. The molar ratio of Al₂ O₃ to MgO was 1:1. The average particle size was submicron. The powder was mixed with 1/2 weight percent LiF. The powder was loaded in a graphite die and cold-pressed at about 100 psi (0.7 MPa). The cold-pressed body was hot pressed at a temperature of about 1410° C. with a uniaxial pressure of about 2200 psi (15 MPa) with a "soak" of 5 hours. After cooling, the bodies were hot isostatically pressed at a temperature of 1500° C. and a pressure of 29,500 psi. After cooling, the bodies were polished. Transmission spectra characteristic of the three bodies are shown in FIG. 10.

Example 2

Bodies of calcium aluminate glass and bodies of spinel, the latter formed according to the process of the invention, as described in Example 1, were subjected to sunlight and an artificial ultraviolet light source, Model UVL-56 manufactured by Ultra-Violet, Inc. of San Gabriel, Calif., for periods of time indicated in Table 5. The transmission of the bodies at a wavelength of about 0.254 microns is also shown in Table 5 before such exposure to ultraviolet light and after such exposure to ultraviolet light.

                  TABLE 5     ______________________________________            Length            of                Transmission                                        Transmission     Type   Exposure Light    Before    After     of Body            (hours)  Source   U.V. Exposure                                        U.V. Exposure     ______________________________________     Calcium-             2       Sunlight 70        45     aluminate            1/4      Artificial                              70        68     glass     Spinel 244      Sunlight 78        78            115      Artificial                              78        76     ______________________________________

Example 3

To assess the damage from sand erosion, samples of material formed according to the present invention, along with plexiglass and window glass were mounted on a rotating table and blasted with 10 pounds of abrasive grit. The percent haze after the test was measured using an integrating sphere. The results are shown in Table 6.

                  TABLE 6     ______________________________________                       Percent Haze     Material          After Sandblast Test     ______________________________________     Spinel - Hot press + HIP                        8     Plexiglass - 1    79     Plexiglass - 2    74     Window Glass      82     ______________________________________

Example 4

The Agile Rain Erosion Test, as described in Memo REG. 4062-137-72 "Rain Erosion Test of Proposed Dome Materials" was performed on the material produced according to the present invention as well as a number of other materials including MgF₂, CaF₂, Yttrium-Aluminum-Garnet, MgO, and two glasses, Vycor 7913 and Corning 1723. The materials were laced in a sample holder and attached to an accelerated sled achieving the velocities indicated in Table 7. Results of the test are shown in Table 7. Table 8 shows the resistance to damage of the materials tested in order of relative effectiveness.

                  TABLE 7     ______________________________________             Thickness             of Al.sub.2 O.sub.3                              Velocity             Coating   Run    (km per     Material             (microns) No.    hr/fps)                                     Description     ______________________________________     Spinel- None      1      29/1600                                     No appreciable     hot-press +                     damage     HIP             None      3      25.5/1400                                     No damage             None      4      29/1600                                     No damage     MgF.sub.2             None      1      29/1600                                     Completely                                     shattered             None      2      29/1600                                     Completely                                     shattered             None      3      25.5/1400                                     One surface crack             None      4      29/1600                                     Complete breakage             2.1       4      29/1600                                     Surface cracks                                     with coating                                     deterioration             3.9       4      29/1600                                     Coating slightly                                     deteriorated     CaF.sub.2             None      1      29/1600                                     Slight pitting                                     with surface                                     cracks & fractures                                     throughout                                     although sample                                     intact             None      2      29/1600                                     Surface cracks                                     with pitting             None      4      29/1600                                     Surface cracks                                     with slight                                     pitting             2.1       4      29/1600                                     Coating gone -                                     slight pitting                                     with surface                                     cracks     MgO     None      1      29/1600                                     Surface cracks             None      2      29/1600                                     Complete breakage     Sapphire             None      2      29/1600                                     Single line                                     fracture             None      3      25.5/1400                                     No damage     Yttrium-             None      1      29/1600                                     Single line     Aluminum-                       fracture     Garnet             None      3      25.5/1400                                     No damage     Corning           2      29/1600                                     Completely     1723                            shattered                       3      25.5/1400                                     Pitting with                                     surface cracks     VyCor             2      29/1600                                     Completely     7913                            shattered                       3      25.5/1400                                     Surface pitting                                     with one surface                                     crack     ______________________________________

                  TABLE 8     ______________________________________     Agile Rain Erosion Test Resistance To Damage     (In Order Of Relative Effectiveness)     ______________________________________     1              Spinel; Hot-press + HIP     2              Sapphire     3              Yttrium-Aluminum-Garnet     4              MgF.sub.2 with coating     5              CaF.sub.2 with coating     6              MgO     7              CaF.sub.2     8              MgF.sub.2     9              VyCor 7913     10             Corning 1723     ______________________________________

Although the present invention has been described with reference to certain embodiments, it should be appreciated that further modifications can be effected within the spirit and scope of the invention as limited only by the appended claims. 

What is claimed is:
 1. A sintered polycrystalline ceramic body consisting essentially of:magnesia-alumina spinel; said body having an in-line transmittance per 1.88 mm thickness in all wavelengths in the wavelength range from about 0.2 microns to about 0.4 microns of not less than the transmittance shown by line B in FIG.
 2. 2. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has a flexural strength at 25° C. of at least about 15000 psi.
 3. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has a knoop harness of at least about 1350 kg mm⁻².
 4. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has a tensile strength of at least about 12,000 psi.
 5. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has an average grain size less than about 150 microns.
 6. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has an average pore radius of less than about 0.1 microns.
 7. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has a scatter at a wavelength of about 0.6 microns of less than about 9×10⁻³.
 8. A sintered polycrystalilne body, as claimed in claim 1, wherein:said body can be polished to a scratch-dig size of less than bout 20--20.
 9. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has sufficient chemical stability that there is substantially no solubility of said body in an acidic aqueous solution having a pH of between about 2 and
 7. 10. A sintered polycrystalline body, as claimed in claim 1, wherein:said body has sufficient thermal stability that there is substantially no degradation of said body upon exposure to temperatures of about 500° C.
 11. A sintered polycrystalline body, as claimed in claim 1, wherein:said polycrystalline body has an in-line transmittance per 1.88 mm thickness in all wavelengths in the wavelength range from 3 microns to 5 microns of not less than about 73 percent.
 12. A sintered, polycrystalline body as claimed in claim 1, wherein:said body has a flexural strength at 25° C. of at least about 15,000 psi; and said body has a knoop hardness of at least about 1350 kg mm⁻².
 13. A sintered polycrystalline body, as claimed in claim 1, wherein:said body comprises at least 99.9 weight percent magnesia-alumina spinel.
 14. A sintered polycrystalline body, as claimed in claim 1, wherein:said body comprises alumina and magnesia in a molar ratio in the range from about 0.95:1 to about 1.05:1.
 15. A sintered polycrystalline ceramic body consisting essentially of:magnesia-alumina spinel; said body having an in-line transmittance per 1.88 mm thickness in all wavelengths from about 0.3 microns to about 0.8 microns of not less than about 85 percent; and said body retaining at least about 99 percent of its original transmissivity after 200 hours of exposure at a distance of about 15 cm to ultraviolet light of a wavelength of about 0.254 microns and an intensity of about 700 microwatts/cm².
 16. A sintered polycrystalline body, as claimed in claim 15, wherein:said body has a flexural strength at 25° C. of at least about 15000 psi; and said body has a knoop hardness of at least about 1350 kg mm⁻².
 17. A sintered polycrystalline body, as claimed in claim 15, wherein:said body comprises at least 99.9 weight percent magnesia-alumina spinel.
 18. A sintered polycrystalline body, as claimed in claim 15, wherein:said body comprises alumina and magnesia in a molar ratio in the range from about 0.95:1 to about 1.05:1.
 19. A sintered polycrystalline body consisting essentially of:magnesia-alumina spinel; said body having an in-line transmittance per 1.88 mm thickness of not less than about 86% at some wavelength within the wavelength range between about 0.2 microns and about 0.4 microns.
 20. A sintered polycrystalline body, as claimed in claim 19, wherein:said body comprises at least 99.9 weight percent magnesia-alumina spinel.
 21. A sintered polycrystalline body, as claimed in claim 19, wherein:said body comprises alumina and magnesia in a molar ratio in the range from about 0.95:1 to about 1.05:1.
 22. A body consisting essentially of:a sintered polycrystalline magnesia-alumina spinel ceramic; said body having an in-line transmittance per 1.88 mm thickness in all wavelengths from about 0.3 microns to about 0.8 microns of not less than about 85 percent; said body having a knoop hardness of at least about 1350 kg mm⁻² ; and said body having a flexural strength at 25° C. of at least about 20000 psi.
 23. A body, as claimed in claim 22, wherein:said body has an in-line transmittance per 1.88 mm thickness in all wavelengths in the wavelength range from about 0.2 microns to about 0.4 microns of not less than the transmittance shown by Line A in FIG.
 2. 24. A body, as claimed in claim 22, wherein:said body has a melting point above about 2000° C.
 25. A body, as claimed in claim 22, wherein:said body is substantially non-birefringent.
 26. A sintered polycrystalline body, as claimed in claim 22, wherein:said body comprises at least 99.9 weight percent magnesia-alumina spinel.
 27. A sintered polycrystalline body, as claimed in claim 22, wherein:said body comprises alumina and magnesia in a molar ratio in the range from about 0.95:1 to about 1.05:1.
 28. A sintered polycrystalline ceramic body consisting essentially of:magnesia-alumina spinel; said body having an in-line transmittance per 1.88 mm thickness in all wavelengths from about 0.3 microns to about 0.8 microns of not less than about 80 percent.
 29. A sintered polycrystalline body, as claimed in claim 28, wherein:said body comprises at least 99.9 weight percent magnesia-alumina spinel.
 30. A sintered polycrystalline body, as claimed in claim 28, wherein:said body comprises alumina and magnesia in a molar ratio in the range from about 0.95:1 to bout 1.05:1.
 31. A sintered polycrystalline ceramic body consisting essentially of:magnesia-alumina spinel; said body having an in-line transmittance per 1.88 mm thickness at some wavelength from about 0.2 microns to about 0.3 microns of not less than about 72 percent.
 32. A sintered polycrystalline body, as claimed in claim 31, wherein:said body comprises at least 99.9 weight percent magnesia-alumina spinel.
 33. A sintered polycrystalline body, as claimed in claim 31, wherein:said body comprises alumina and magnesia in a molar ratio in the range from about 0.95:1 to about 1.05:1.
 34. A sintered polycrystalline ceramic body consisting essentially of:magnesia-alumina spinel; said body comprising alumina and magnesia in a molar ratio from about 0.95:1 to about 1.05:1 wherein said body has an in-line transmittance per 1.88 mm thickness in all wavelengths in the wavelength range from about 0.2 to about 0.4 microns of not less than the transmittance shown by line A in FIG.
 2. 